Hum In My Amp

[Dmoney]

HEY!
so I built this and made a thread about it. In that thread I mentioned that it had some noise going on, but you couldn't really hear it on the clip. Anyway, here is a gut shot...



Here is some stuff I've tried to track down the problem...

Test: Removing FX loop, Line Out, and Bypassing/Removing V4
Result: No Change

Test: Pulling 12AX7s Starting with V1
Result: Noise Decreases and disappears when phase inverter is removed

Test: Using different power & preamp tubes
Result: No Change

Test: Grounding preamp at star
Results: No Change

Test: Grounding Preamp at Power PCB
Result: No Change

Test: Grounding Preamp at footswitch Jack (footswitch jack sleeve acts as ground for the switching supply)
Result: No Change

Test: Lifting Footswitch Jack from chassis ground
Result: Introduces some hum

Test: Grounding filament centre tap at star rather than potential divider (Elevated heaters where in from the start)
Result: No Change

Test: Grounding Output Jacks at the chassis
Result: No Change

Test: Moving wires using chopstick & replacing shielded wire
Result: No Change

Test: Altering position of Power PCB & Choke
Result: No Change

Test: Adding a 47uf/450V cap in parallel with C33 (part of the filtering)
Result: No Change

Test: Adding a 200uf/300V cap in parallel with each AC Mains filter cap (these work in pairs. Two before the standby switch and choke, and 1 pair after)
Result: No Change

Test: Pot case grounding at chassis improved
Result: No Change

Test:
1) re-wetting EVERY joint in the amp. Heated it and put a touch of solder to it. Inspected it. etc...
2) Removed unused components (PCB mounted FX Loop related parts)
3) Changed AC bridge rectifier diodes (not the bias diode) from UF5408 to some spare 1N4007's
4) Changed the power tube grid stops to 2.2K from 5.6K (as per my EL34 Avenger)
5) Tried using totally different tubes again.
Result: No Change. Still get the same noise. Changing diodes at this point didn't make a noticeable difference.

Test: Tried charging the caps up while playing and then continuing to play with the mains off and standby on.
Result: The hum did vanish from the output.




Any more ideas? It's Oscilloscope time, right?

[martinw]

Have you scr@ped the powder coating off at the earthing points?

[Dmoney]

yeah thats been done.
thats what was 'improved' on the pot case grounds.
The locking washers didn't pierce the powder coat there, but everything like that is cool.

all the voltages are what they should be.

[martinw]

Are the preamp heaters  AC? If so, try running them on a 5V DC supply. If the hum goes, it proves the heaters are the problem. (Looking at the way those rails are set up, I hope they are DC!)

It also might be an earth loop. Make sure each part of the amp is only grounded in one place (lift the ground and measure for an open circuit.)

The fact that the hum goes when you take the PI valve out shows its in the preamp.
The fact that the hum goes immediately you turn off the mains shows that it is coming from an AC source isolated by the mains switch.

Keep plugging away, you'll find it!  :)

[Dmoney]

believe it or not, the real SLO has heater wires like that, and its not DC powered. It's normal AC heaters.
These are just DC elevated. Report from people who have built the amp the same way say their amps are super quiet, so they are either lying and the SLO is just weird, or it can actually work.

I understand why you usually twist heater wires though.

I'm trying to find a 5vdc power source.
the other AC source in there separate (separate from the heaters) source for the LDR's.

[martinw]

I know it can work, I have seen it on other amps.  :)

Does the hum increase with gain or volume?

[Dmoney]

yeah it increases with volume and gain.

[Dmoney]

RIGHT!

so, I figured out a way!

I got an Ipod wall wart charger, and butchered a USB cable, twisting together to data wires and leaving the power and ground separate.

The ipod charger kicks out 5VDC 1000mA.

I detatched the AC heater wires and taped the ends individually and pulled them out the way.
I plugged the USB cable into the charger after measured the voltage on my DMM, 5.4VDC.
Unplugged the USB cable, and wired the +5V red USB wire to pin 4&5 rail, and the ground to the pin 8 rail.
I only put in the phase inverter, which does have the noise present if in place by itself, but mainly because of the current draw from the heaters in the 12AX7's. I think all 4 12AX7's would overload the charger.
I plugged the USB back in and got GLOWING FILAMENTS!

I turned the amp on and got some hum, but different to the hum I had previously.
It was loud (I guess powering your preamp using a banged up USB cable and an ipod charger isn't so good)

When I turned the amp off BOTH mains and standby, I got some REALLY loud hum coming out the speakers.

probably not a good, or definitive test (I doubt the filtering on my ipod charger is very good) but it did alter the characteristics of the hum in some way.


and yes, I walk a fine line between genius and stupidity.

[martinw]

If the original noise has definitely gone (as opposed to just being swamped) then that's a good sign that the hum was coming from the heaters.

The other part of the equation is finding what's picking it up. If that heater wiring style works ok in most SLO's, then you wuld expect the problem to be a signal wire that is too close to the heaters, or runs parallel to them for too long, or possibly that the heater supply itself is noiser than it should be.

Have you definitively eliminated earth loops in the preamp area, including the i/p jack?

[Dmoney]

the grounding is a kind of mix of bus and star.

the power pcb has one point to ground that goes to the star.
the High Voltage centre tap is at the star
the PT ground is at the star
a rail that joins Pin 1 on all power tube sockets is at grounded at the star
(power Pin 8's are joined in pairs, then each pair to a 1K/3W resistor, then to the pin 1 rail via bias test points)
Pot Casings are grounded at the chassis. No bus on the back of the pots.
output jacks are grounded to the chassis, though both sleeves are wired together.
the OT secondary common is grounded at the output jacks.
the Footswitch jack is grounded at the chassis, this makes the ground for the switching supply.
The input jack is isolated from the chassis but the sleeve sits on the ground lug rail that runs across all the pots that have lugs going to ground. This rail is wired to the preamp PCB, which has a ground track running through its centre, at the other end of which this is joined to the star ground.

the only signal wires that go near the heaters are the depth and presence related wires. but moving them while the amp is on doesn't make any difference. In fact moving any wire while the amp is on makes no difference.

[HTH AMPS]

try referencing the heater CT to around 40vdc - this always kills hum when I've had such a problem.

the easiest way is to create a 40v reference from the HT line via a voltage divider with a cap bypassing the resistor that goes to ground.

[Dmoney]

That's exactly the case in this amp. It has a faux center tap though in this case. Two 100ohm resistors, one on each heater wire, with one end wired to the other on a cable running to the potential divider. This wires is away from anything but other power related wires.

This was a great mod in other amps I've owned

[HTH AMPS]

lets try the easy fixes first...

* swap the preamp valves one by one - they do go bad sometimes.
* how closely are the OP valves matched? - if they're too far apart you'll get hum.

I know the heater wires in the SLO are run parallel (not twisted), but that could be a potential cause of your hum too.

Check your grounding too - in a high gain amp you have to be a bit more organised as random grounding will likely lead to ground loops (unless you hit lucky).  This article on star grounding is worth a read... http://www.aikenamps.com/StarGround.html as is this one... http://www.freewebs.com/valvewizard2/Grounding.pdf

 

[Dmoney]

thats all been checked.

I've swapped out all the tubes various times.
tried 3 sets of EL34's, 2 new, and one from an amp that has no hum problem at all.
did the same with 12AX7s.

That was one of the first things I checked, as I started out using spare valves in this thing.

It may well be the heaters.

I've read the grounding article too, and checked the grounding against the real SLO and the Clone schematic/layout pdf's and detailed photo's of other peoples build that they claim work fine.

most of what I've done is detailed at the start of the post, changing valves out is in there too.

I even opened my Avenger, which is pretty much the same OD circuit and looked at the grounding scheme in that.
As it is now, it's pretty much the same. (EDIT: I don't mean I copied it blindly)


anyway, yesterday I decided that I'd got to the point where I couldn't do any more with what I have, so I took it to JPF Amps, so I'm sure that will be a treat for Frank to look at... Sorry Frank!

truth is, it was depressing me when it was in my line of sight. I needed to do something with it.
SO close!

[BigB]

truth is, it was depressing me when it was in my line of sight. I needed to do something with it.
SO close!

Won't be of any technical help, nor probably of much help at all, but as far as I'm concerned, bringing such a project "SO close" would be an outstanding achievement (and that's still an understatement).

So blow away your bluesy feelings
Spirit says, take the world off your shoulders...